The growth and maturation of the fetus in utero. This is divided into three periods: the preembryonic period begins with conception and ends on gestational day 14; the embryonic period encompasses gestational weeks 3 through 8; and the remainder of the pregnancy is known as the fetal period. Body organs and systems arise from three primary germ layers (ectoderm, mesoderm, and endoderm) and rudimentary formation of all organ systems is completed by gestational week 16. Systems maturation essential to extrauterine survival begins during week 24 with the formation of pulmonary surfactant. Two critical events occur between weeks 26 and 29: the pulmonary vasculature becomes capable of gas exchange and the central nervous system becomes capable of controlling respiration.